LCD having a compensator directly on the substrate of adjacent LC cell having particular subpixels

ABSTRACT

In a normally black double cell, grey scale enhancement is obtained by dividing pixels in the driving cell into sub-pixels which are rotated preferably through 180° with respect to each other.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a liquid crystal display device having, betweena polarizer and an analyzer, a first layer of twisted liquid crystalmaterial with a twisted structure between two transparent substrates,with pixels being realized between the substrates, and a compensatorlayer. Such display devices are generally used in, for example,automotive displays, but also in monitors, etc.

2. Description of the Prior Art

An example of such a display device is shown in, for example, U.S. Pat.No. 5,287,207. This patent shows a double cell in which a second cellfunctions as a compensator in order to obtain an optimum contrast.Although a satisfactory contrast is obtained in these types of displaydevices upon perpendicular passage of the light, it appears that thereis a considerable grey scale inversion in such a double cell, whenviewed at an angle.

BRIEF SUMMARY OF THE INVENTION

It is, inter alia, an object of the present invention to reduce oreliminate said grey scale inversion.

To this end, a display device according to the invention ischaracterized in that a pixel comprises at least two sub-pixels havingthe same twist and, viewed transversely to the substrates, twist angleswhich are rotated with respect to each other. The twist angles, viewedtransversely to the substrates, are rotated substantially 180 degreeswith respect to each other. The effects on the grey scale of onesub-pixel are then compensated, as it were, by the effects of the othersub-pixel. Also in the case of rotations different from 180 degrees, anenhancement is obtained dependent on the type of usage (for example, incockpit applications or in applications where a display device is viewedby different persons from two directions).

The twist angles are preferably in the range between 50 and 100 degrees.

A first preferred embodiment of a liquid crystal display deviceaccording to the invention is characterized in that the compensatorlayer has a twisted structure with a twist which is opposite to that ofthe layer of twisted liquid crystal material. The compensator layerpreferably has a twist angle which is opposite to that of the layer oftwisted liquid crystal material.

Further enhancements are possible by adding a retardation layer with anoptical axis perpendicular to the compensator layer.

It is also possible to use negative birefringent materials such asdiscotic materials for the compensator layer.

These and other aspects of the invention are apparent from and will beelucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a cross-section of a display device according to theinvention,

FIG. 2 shows diagrammatically some parts of the display device accordingto the invention,

FIG. 3 shows diagrammatically different orientation directions in thedisplay device of FIG. 1,

FIGS. 4 and 5 show the luminance as a function of the viewing angle fora double cell without using and using, respectively, the measureaccording to the invention, while

FIG. 6 shows diagrammatically different orientation directions, and

FIGS. 7 and 8 show the luminance again as a function of the viewingangle for a double cell in a display device with a compensation cellbased on discotic material without using and using, respectively, themeasure according to the invention.

The drawings are diagrammatic and not drawn to scale. Correspondingparts are generally denoted by the same reference numerals.

DETAILED DESCRIPTION OF THE INVENTION

The display device of FIG. 1 comprises a first display cell 10 with alayer 11 of a liquid crystal material having a positive dielectricanisotropy between transparent supporting plates 2, 3. On the side ofthe liquid crystal material, electrodes 12, 13 defining, for example, amatrix of pixels are present on the supporting plates 2, 3. The pixelsmay be driven directly via row and column electrodes (positive drive).In this example, the pixels are defined by overlapping portions ofpicture electrodes 12 and a counter electrode 13, in which the pictureelectrodes are provided with pixel voltages by switching elements (notshown) via row and column electrodes (active drive). The drive voltages(selection and data signals) are obtained, for example, by means of adrive circuit 7 which converts incoming information 8 into said drivevoltages which are applied to the electrodes 12, 13 via the switchingelements (not shown) (here diagrammatically shown by means of switch 15in one of the connection lines 16, 16′). Layers 14 of an insulatingmaterial, which in this example also serve as orienting layers, arepresent on the layers 12, 13. The layers 14 give the liquid crystalmolecules a twist angle Φ₁ at a voltage of 0 volt across the electrodes12, 13. The twist angle Φ₁ is between 50° and 100° and, in this example,90°.

The device further comprises a compensator layer, in this example, asecond cell 20 with a second layer 21 of a liquid crystal materialhaving, in this example, also a positive dielectric anisotropy betweentwo transparent supporting plates 3, 4. The supporting plate 3 is chosento be common for both cells 10, 20, but this is not strictly necessary,although it simplifies the manufacture in which also the double cellmaintains a small total thickness. Layers 24 for insulation andorientation are present on the supporting plates 3, 4. The liquidcrystal material 21 and the orienting effect of the layers 24 are chosento be such that the liquid crystal molecules acquire a twist angle Φ₂opposed to Φ₁. The direction of orientation in this example is the samefor the layers 14, 24. The cells 10 and 20 are situated between apolarizer 6 and an analyzer 5 whose directions of polarization aremutually crossed perpendicularly.

According to the invention, parts 14′, of one orientation layer within apixel are provided in such a way (for example, via masked rubbing orvapor deposition) that the direction of orientation is substantially180° different for the liquid crystal molecules in the two sub-pixels.This is further shown in FIG. 2 in which the polarizer 6, parts 14′, ofan orientation layer on the supporting plate 2, the orientation layers14, 24 on the supporting plate 3, the orientation layer 24 on thesupporting plate 4 and the analyzer 5 are shown diagrammatically. Thedirections of orientation of the parts 14′, of the orientation layer onthe supporting plate 2, the orientation layers 14, 24 on the supportingplate 3 and the orientation layer 24 on the supporting plate 4 aredenoted by the reference numerals 27′, 27″, 27, 28 and 29, respectively.The directions 27, 28 cross each other substantially perpendicularly.

The absorbing axis 26 of the polarizer 6 extends at an angle of 0°(180°) to the orientation direction 27′ (27″) in this example. Alsoother angles between the axis 26 and the direction 27 are possible. Thewall orientation of the layers 14, 24 is chosen to be such that thedirector in the center of the cell 10 (at 0 volt) is perpendicular tothe director in the center of the cell 20. Due to the division thuschosen, two twist angles Φ₁ are introduced in sub-pixels of the cell 10,which angles, viewed transversely to the substrates, are rotated throughsubstantially 180 degrees with respect to each other. This is showndiagrammatically in FIG. 3. The twist angles of the sub-pixels aredetermined by orientation directions 27′, 27 and 27″, 27 for the cell10. In one sub-pixel, polarized light (polarized in the direction 26 ofpolarization of polarizer 6) is twisted through an angle Φ₁ fromdirection 27′ to direction 27 at 0 volt, and in the other sub-pixel fromdirection 27″ to direction 27. In the second cell 20 (twist angle −Φ₁),the directions of polarization for both sub-pixels are twisted backthrough the same angle from direction 28 (parallel to direction 27) todirection 29, so that it does not pass the analyzer 5 with the passdirection 25 (normally black).

For the grey scales as a function of the viewing angle of this doublecell, it now holds that it is composed, as it were, from those of thetwo sub-pixels. Since these are symmetrically situated with respect toeach other, a symmetrical characteristic is obtained. This is shown inFIGS. 4 and 5. FIG. 4 shows the characteristic curve of the luminance asa function of the viewing angle for a double cell without said measure(with a twist angle of 60° and d.Δn=330 nm) for different drivevoltages. Grey scale inversion already starts at a viewing angle of 20°and occurs for all grey values. FIG. 5 shows this characteristic curvefor a double cell, using said measure. Grey scale inversion now startsat viewing angles of more than 50° and then only for some grey values.

In the device shown diagrammatically in FIG. 6, the twist angle Φ₁ ofthe cell 10 is 90° and d.Δn=400 nm. In this example, the second cell 20has a second layer 21 of a liquid crystal material with a negativedielectric anisotropy. The orienting effect of the layers 24 is chosento be such that the liquid crystal molecules acquire a twist angle of−Φ₁. Such a cell 20 is obtained, for example, by choosing a discoticliquid crystalline material for the liquid material. In this example,the layer 21 is liquid but it may also consist of a polymerized liquidcrystalline material. The orientation layers 24 are adapted in such away that the directors of the discotic molecules acquire an orientationdirection 28 at the area of substrate 3, transverse to the orientationdirection 27 of the (calamatic) molecules at the area of substrate 3.

FIG. 7 shows for this double cell the luminance as a function of theviewing angle for a double cell comprising a compensation cell 20 whichis based on a discotic material, without using said measure, and fordifferent drive voltages, and FIG. 5 shows the characteristic curve forsuch a double cell, using the measure according to the invention.

The invention is of course not limited to the examples shown. Forexample, the compensation cell 20 may comprise a foil of polymerizedliquid crystal with a twisted (director) structure instead of a liquidcrystal material.

In summary, the invention yields an improvement of the grey scale in“normally black” double cells by dividing a pixel in the cell to bedriven into sub-pixels which are rotated preferably through 180° withrespect to each other.

The invention resides in each and every inventive characteristic featureand each and every combination of characteristic features.

What is claimed is:
 1. A liquid crystal display device having, between apolarizer and an analyzer, a first layer of twisted liquid crystalmaterial with a twisted structure between two transparent substrates,with pixels being realized between the substrates, and a compensatorlayer disposed directly on one of the transparent substrates and havinga twisted structure with a twist opposite to that of the first layer ofliquid crystal material, wherein a pixel has at least two sub-pixels andat least a part of an insulating and/or orientation layer associatedwith said pixel is provided such that the direction of orientation issubstantially 180° different in said at least two sub-pixels.
 2. Aliquid crystal display device as claimed in claim 1, characterized inthat the compensator layer has a twist angle which is opposite to thatof the layer of twisted liquid crystal material.
 3. A liquid crystaldisplay device as claimed in claim 1, characterized in that thecompensator layer comprises at least a retardation layer with an opticalaxis perpendicular to the compensator layer.
 4. A liquid crystal displaydevice as claimed in claim 1, characterized in that the compensatorlayer comprises a negative birefringent material.
 5. A liquid crystaldisplay device as claimed in claim 1, characterized in that thecompensator layer comprises a liquid crystal material with a twistedstructure.